As the size of transistors in integrated circuits (ICs) continues to decrease, the power supply voltage to the transistors and capacitance of the transistors also decrease. As the power supply voltage decreases, the threshold voltage of the transistors in the ICs must also decrease.
Lower threshold voltages are difficult to obtain in conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) because, as the threshold voltage is reduced, the ratio of on-current to off-current (Ion/Ioff) also decreases. The on-current refers to the current through a MOSFET when a gate voltage applied is above the threshold voltage, and the off-current refers to current through a MOSFET when a gate voltage applied is below the threshold voltage.